Telephone system



' Jan. 6, 1942. G. HECHT EI'AL TELEPHONE SYSTEM I Filed Sept. '7, 1940 a. HECHT A. A. LUNDST/POM /N l E N TORS:

ATTORNEY phone system of the type in which automatic 3 Patented Jan. 6, 1942 TELEPHONE sY's rEMI George Hecht, Astoria, N. Y., and Alexis A.

' Lundstrom, East Orange, N. .L, assignors to Bell Telephone Laboratories, Incorporated, New

York, N. Y., a'corporation of- New York.

Application September 7, 1940,Serial No; 355,712

5 Claims. gel; 173-18) Thi invention relatesfito telephone systems" and particularly to the receiving equipment of a transmission line over which alternating currents, preferably within the voice. frequency range, are transmitted for effecting the operation of said equipment which controls, in turn, the selective positioning of switching apparatus. More particularly, the invention'is in thenature of an improvement in the receiving equipment of the telephone transmission line disclosed and claimed in the copending application, of A. A. LundstronSerial No. 306,466,-filed November 28, 1939, now Patent'No. 2,237,742, granted April 8,

In said application 'is disclosed a teleswitches are controlled'bymeans responsiveto alternating currents of different frequenciesgening device.

the channel detectors to function, it is possible to disable the channels after complete operation but before the entire duration of the current impulse is exhausted and thereby" protect those channels which have not operated from falsely operating on signal currents which may be falsely produced on the release of a key at the generat- Accordingly, this feature of the invention includes as apart of the-means which activate the channel:de'tectors, a timing device which is adapted to keep said channel detectors erated at a subscriber's station and transmitted over a line which is provided-with a plurality of channel detectors each selectively tuned to a different one of the frequencies and responsive thereto to operate arelay which,'j in combination with a relay operated overanother-channeldetector, serves to function a'settable register, or sender, to lock in a; record of the digit indicated by the two operated relays; that is, by the two frequencies transmittedirom the subscribers station. j p 5 In systems of this kind'wherethe signal :generating instrument is, usually, a plurality of keys or the like, the operation of one of said keys to set one or more reeds vibrating in amagnetic field to produce the currents of desired frequencies, is liable to produce a jagged-cut-off of the vibrating reeds when they comeunder the action of dampers upon the release of the key. The result is that undulating currents are produced in response thereto which, containing components of the difierent frequencies to which the channel detectors are respectively tuned .to operate, are liable to cause said channel detectors to respond falsely'to said currents and thereby producea false registration. I 1

In'accordance with -a feature of this. invention, the channel detectors are normally kept unresponsive until the first impulse of current is received from the generating device at which time circuit means are activated in response'to said impulse which apply a sensitizing potential to the cathodesof theindividual detector, tubes in the several channels, which are thereby activated to respond to the remainder of the current impulse. Since the length of the impulse produced by the depression of a key at, the generating device, is longer than necessary'to permit H activated for a predetermined intervaL'measured from the instant the signal .arrives, and is then operated .to desensitize the detector channels to make them thereafter unresponsive to the signal notwithstanding, thev fact that the signal current isstillontheline.

This and other features of the invention will be more readily ascertained from the following detailed description of said inventiomappended claims and attached drawing in which is shown schematically certain elements of the telephone system'more 'completely disclosed in the abovementioned copending application of A. A. Lundstrom and in which certain other elements to which our invention more directly relates, are shown in detail.;- j

Referring, now, to the drawing, A represents a subscribers station in an automatic telephone system, said'station being equipped with; a-device 1| suitablyarranged to. produce one or more currents of different frequencies in responseto the depression. of keys-or the like that represent digits or other characters making up the wanted .designation-; 2 represents a line extending from the station instrument to the central ;oifice whence, through one of a plurality of selectable senderconnectors [3, the line is extended to a free sender which comprises coupling transformthe channels flf 5 are completely disclosed in the above-mentioned copending application of A, A. Lundstrom to which reference is made for a more complete understanding thereof although it is to be understood that the invention is not limited to channels of the type described therein.

In the operation of the invention, a call is initiated at station A by the removal of the receiver from the hook whereupon the line is then extended by any suitable means to the central office at which an available sender is then connected to said line through the medium of a sender connector I3, after which key tone is transmitted back to the calling station A as a signal that the sending device I may be operated to transmit the number of the wanted station, all in the manner described in the above-mentioned copending application.

When any one of the ten keys of the sending device I is operated, two out of the five defined frequencies flf are produced in the combination required to designate the digit or character represented by the key and transmitted over a circuit which may be traced from one side of thesending device. I, lower conductor of the looped pair 2, lower back "contacts of relay [2, next to'the inner contacts of sender connector cut-in relay I3, primary winding of transformer '3, inner contacts of relay I3, top outer'back contacts of relay I2, upper conductor of looped pair -2' to the other side of the station sending device I. The current composed of the'two frequencies is induced in the secondary winding of transformer 3 and fed into itheinput side of amplifier 8 whence, after suitable amplification, it is applied, through transformer 4, to the input side' of the volume limiter II and is also applied, through tr'ansformerfi, to amplifier .9 whence, aftersuitable amplification, it is applied, via transformer 6, to the enabler circuit I0.

} Considering that portion of the slgnalenergy which passes into the volume limiter II, the energy causes said-volume limiter to operate and 'regulate the energy output in accordance with its adjustment which may be such as to limit the-magnitude of output to that required for the satisfactory operation of'the channel detectors" and no more, regardlessof the magnitude of the input energy. The reason for doing this is, of course, to prevent overloading any of the channel detectors which, should it occur in the event of a heavy input signal due, say, to'a very short loop 2, would cause; false operation of ad- 'joining channel detectors by currents present in the frequency spectrum generated thereby and thus cause the registration of a false number. Hence by adjusting the volume limiter to a suit able level of output energy above the operate level of the channel detectors, their false operation'upon heavy line currents is avoided.

The energy output from the volume limiter I I is applied, through transformer 1, to the variable resistance pad I4 whence it is applied through suitable filtering devices (not shown) to the grids of the detector tubes, in each of the channels. These tubes, however, are normally inactive due to the absence of activating potential upon the cathodes thereof, which potential is applied over conductor I5 as explained hereinafter. The channels that would be involved by the two frequency components in the current transmitted through variable resistance pad I4, therefore, do not operate.

Considering, now, that portion of the signal energy'which is transmitted to the enabler circuit III via transformer 6, it will be observed that'said enabler comprises a rectifier bridge l6 and four polarized relays I'I, I8, I9 and 20,

each having a permanently closed polarizing circuit through its lower winding which causes the armatures thereof to assume the position indicated in the drawing. Further, between each two relays, beginning with relay I1 and ending with relay 20, there is an intermediate condenser-resistance network which joins a contact of the relay on the left with the upper winding of the relay to the right. Considering relays I1 and I8, for instance, the network made up of resistances 22 and 23 and condenser 2| joins the right contact of relay H with the upper winding of relay I8. Since the armature of relay I1 is connected to positive battery 30, and condenser 2| is connected to ground, said condenser is normally charged through resistance 22 to the potential of said battery. Condenser 24 of the network between relays I8 and I9, on the other hand, is normally at ground potential since it has ground applied to it through the left contact of relay I8 and resistance 25. Contrariwise, condenser: 29 of. the network between relays I9 and 20 is charged to the potential of the battery 38' through the upper winding of relay 20 since the short-circuiting ground path for condenser 29 is normally open at the right contact of relay i8, thereby leaving condenser 29 exposed to the charging circuit that traces from the positive pole of battery 30,- upper winding of relay 20, resistance 28, condenser 29 to.ground.

Assuming, therefore, that relays 11, I8, I9 and 20 have their respective armatures in the position shown, and.that condensers 2|, 24 and 29 are charged, discharged and charged, respecof the several channel detectors fI-j5, and

causes them to be normally unresponsive to signal energy incoming over the resistance pad I4 as above described.

Now when" a portion of the incoming energy is applied to the rectifier bridge 16, said energy is rectified and thecurrent thereof is applied to the upper winding of relay H. The combined effect of the current now flowing through the upper winding of this relay and that normally flowing through its lower winding is such' as to cause its armature toswing from the right contact into engagement with its unwired left contact; Since condenser 2I is normally charged to the potential of battery 30 throughthe armature and right contact of relay H, the opening of these contacts immediately causes the condenser to discharge to ground through resistance 23 and the upper winding ofrelay I8, thereby prolonging the flow of current; that is normally flowin through this relay winding over the circuit traced from battery 30, armature and right contact of relay I'I', resistances 22 and 23, upper winding of relay I8 to ground. The direction of the discharge current as well as the normal current through the upper winding of relay I8 opposes "that of the current normally flowing through its lower winding, and since the discharge current is initially of greater intensity through the suitable choice of the ohmic value of resistance 23, the release of relay'I8 is delayed and the armature thereof is caused to break away from its left contact at a time subsequent to the operation of relay I] and to engage its :right contact. V

During thetime that the] armature of relay I8 is engaged With'its rightcontact, and rememberingthat condenser24 is kept at ground potential while the armature of this relay is making with its left contact; then, as soon as the armature breaks with said left contact, condenser 24 immediately begins t charge over a circuit traced through resistance 26; the upper winding of relay I9 tob'attery 30 to continue to maintain the current flow over this path for a short time subsequent to the operation of relay l8, whereupon the effect of the current through its lower winding continues to be neutralized for a short time bythis changing current rather than by the normal current and the armature of relay I9 is thus caused to break with its left contact at a. time subsequent to the operation of relay I8.

At the instant that the armature of relay l8 engages its right contact and before the armature of relay I9 disengages its left contact, a discharge path is completed for condenser 29 which traced from ground, condenser 29, resistance 21,.armature and left contact of relay I9, armature and front contact of relay I8 to ground. The energy accumulated on condenser 29 as a result of the chargingvcircuit through the upper winding of relay 20 is quickly drained off through the low resistance 21 and a current flow is quickly established in the upper winding of relay 20 to cause its rapid operation and close ground onto conductor I to sensitize the'detectors, so that when the armature of relay I9 breaks with its left contact, condenser 29 is in an uncharged condition and immediately starts recharging through the upper winding of relay 20 to prolong the current flow therethrough for a subsequent period. This charging current overcomes the effects of that normally flowing through lower winding of relay 29 and causes the armature thereof to engage its right contact, thus applying ground to conductor I5 which, as said before, is connected to the cathode of each of the detector tubes in the channel detectors ,fI-f5. The channel detectors are now made responsive and the two which are adapted to respond to the two frequencies in the input current that flows into their respective input side through resistance pad I4 operate, in turn causing the operation of their respective anode relays which, in their turn, cause the operation of relays in the register which designate the digit or character designated by the two frequencies.

It is to be noted that the armature of relay I8 will make with its right contact at a time subsequent to the energizing of relay I 1 by the current through the rectifier bridge Hi. This time is, of course, determined by the discharge time of the condenser 2| through resistance 23 which causes the normal current through relay I8 to endure for a time (let us say, for example, 20 milliseconds) after the operation of relay I'I. At this time relay I8 releases and its armature engages its right contact whereby condenser 29 is quickly discharged through the low resistance 21 and establishes a, current through the upper winding of relay 20 to cause its immediate operation.

This, as explained, places ground on conductor I5 to sensitize the detectors at a time 20 milliseconds after the signal was first applied. This delay is for the purpose of allowing the unwanted spectrum energy in adjoining channels to dissipate itself in order to avoid unwanted channel operation from this source. It is the purpose of release,

the condenser 24 to maintain by its discharge current through the upper winding of relay I9 the normal current flowing therein to hold the armature of relay I 9 engaged with its left contact for a timesuflicient to permit the complete discharge of condenser 29 through resistance 21, contacts of relays l9 and I8 to ground. Relay I9 then operatesto break its left contact and thereafter relay 20 is maintained on the charging current from battery 30, top winding of relay 20, resistance 28, to condenser 29. When this discharge current has dissipated itself, relay 20 will operate and cause its armature to break with its right'contact to remove ground from conductor l5 regardless of how long thereafter that energy is delivered to relay H by the signal.

The length or time during which the armature of relay 20 engages its right contact to cause the channel detectors to remain activated depends, of course, upon theconstants of condensers 24 and 29. and. resistances 26' and 28, which may be chosen to provide any time "interval desired. When the condenser 29 is completely charged, the current through'the lower winding of relay 20 again becomes effective, the armature of the relay breaks away from its back contact and ground is removed from conductorl5 and, therefore, from the cathodes of the detector element of the several detector channels which, in consequence, will no longer respond to any signal energy forthcoming over resistance pad I4 and thereby cause their separate anode relays torelease.

The circuit conditions prevailing in theenabler during the presence of signal energy in bridge l6 and afterthe armature of relay 20 has disengaged its front contact, is that relay II will have its armaturemaking with its left contact, relay I8 will have its armature making with its right contact, while relay l9 will have its armature disengaged from' its left contact which will in no way effect the charge on condenser 29 or the condition of the armature of relay 20 which is out of engagement with its right contact. Hence it makes no difference how long signal energy prevails upon the line subsequent to the armature of relay 20 breaking away from its right contact, said energy will be ineffective to operate the channel detectors inasmuch as ground has been disconnected from the cathodes of their several detector elements.

When the signal impulse is over, relay II will reestablishing quickly thereby the charging circuit of condenser 2| and the current through the top winding of relay I8 to cause the armature of said relay to make with its left contact. Relay I8 operates and causes the quick establishment of current through the top winding of. relay I9 to cause the armature thereof to quickly make with its left contact. In this operation the condenser 29 has not been charged since ground via the contacts of relays I8 and I9 has not been removed. The enabler circuit is now ready for another cycle of identical operations upon the reception of the next impulse.

While we have described our invention and the means for utilizing the same in connection with its specific application to a particular kind of a transmission line, it is to be understood that various other applications and embodiments thereof may be made by those skilled in the art without departing from the spirit of the invention as defined within the scope of the appended claims.

What is claimed is:

1. In a telephone system, the combination with atransmission line comprising a detector channel adapted for response to energy incoming over said lin but normally unresponsive thereto,

means responsive to incoming energy for activating said channel to respond to said energy, and means for maintaining said detector channel activated for a predetermined period while said incoming energy is upon said line regardless of the duration of the energy upon said line after the termination of said predetermined period.

2. In a telephon system, the combination with a transmission line comprising a plurality of channel detectors each adapted for response to a specific frequency of a plurality of frequencies present in a signal current transmitted over said line but normally unresponsive thereto, means responsive to a signal current for activating said channels for response to their respective frequencies in said signal current, and means for maintaining said channel detectors activated for a predetermined period while said current is upon said line whereby they will fail to respond to their respective frequencies in said signal current after the termination of said predetermined period.

3. In combination, a transmission line terminating in a plurality of channel detectors each adapted for response to a specific frequency of a plurality of frequencies in a signal current transmitted over said line but normally unresponsive thereto, means responsive to a signal current for activating each of said channel detectors for response to their respective frequencies in said signal current, said means including other means for keeping each of said channel detectors activated for a period less than the period during which said signal endures upon the line.

4. In a telephone system, a transmission line terminating in a plurality of channel detectors each adapted for response to a specific frequency of a plurality of frequencies present in a signal current transmitted over said line, and means responsive to said signal current for activating, said channel detectors for response to their respective frequencies in said signal current for a predetermined interval, said means comprising a rectifier coupled to said line, a plurality of relays cascaded from said rectifier of which the first is responsive to said signal current after rectification by said rectifier, the last of which applies activating potential to said channel detectors for a predetermined interval, and impedance networks intermediate said plurality of relays for controlling their operating time to make up said predeter mined interval.

5. In a telephone system, a transmission line terminating in a plurality of channel detectors each adapted for response to a specific frequency of a plurality of frequencies present in a signal current transmitted over said line, and means responsive to said signal current for activating said channel detectors for respons to their respective frequencies in said signal current for a predetermined interval, said means comprising a rectifier coupled to said line, four polarized relays cascaded from said rectifier of which the first is responsive to said signal current after rectification by said rectifier, the fourth of which applies activating potential to said channel detectors for a predetermined interval, and a resistance condenser network intermediate adjacent ones, said relays beginning with the first relay and ending with but including the fourth relay for controlling the operating time of all relays except the first to make up said predetermined interval.

GEORGE HECHT. ALEXIS A. LUNDSTROM. 

